Low work function electrodes are highly sought after as they can significantly advance and enable technologies that rely on electron transfer in devices such as electron sources for communications and in particular direct energy converters that transform heat into electricity without mechanically moving components. Electron sources utilizing thermionic electron emitters are widely deployed in high power/high frequency communications (travelling wave tubes, TWT's), radar, free electron lasers, directed energy weapons, X-ray sources and space propulsion. Conventional electron sources based on metallic cathodes operate at temperatures exceeding 1000° C. Lowering the operating temperature would lead to a less involved design, a reduced power demand, and a lighter and smaller payload for operation in mobile terrestrial and satellite applications.
Thermionic energy converters operate through the generation of an electron emission current from a thermionic electron emitter or cathode which is held at a temperature optimized for its emission barrier or work function, typically in excess of 1000° C. for refractory metal based emitters. A second lower work function electrode is coupled to the thermionic emitter, through a small vacuum gap, which establishes a configuration that can generate electrical power. The efficiency can then directly be related to the work function of the counter-electrode, the collector, where an ideal value of 0.5 eV was reported. This ultra-low work function would enable predicted efficiencies greater than 50%. To achieve a similar efficiency with solid-state thermo-electric conversion would require a material with ZT˜10. However, the current best materials exhibit ZT˜2. It is notable that traditional thermal power plants can be characterized as operating with ZT˜3. Additionally, establishing a means to control the electrode work function would enable devices to operate with optimum performance at the desired temperature.